Definitions:


Cement Modified Soils (CMS)
The use of cement or cement at low percentage rates produces an improved existing soil which remains pliable even after compaction and hydration has occurred. CMS reduces a soils ability to retain moisture, and permanently reduces it's Plasticity Index, while simultaneously increasing it's strength. CMS provides a permanent modification to soil which improves strength, the plasticity index and shrinkage limits. CMS is usually added at a low rate of 3% by weight. The depth and percentage of this additive is generally determined by a soils engineer. Spreading and mixing methods will vary as to the lifts, equipment available, and environment.

Cement Kiln Dust (CKD)
CKD reduces the moisture content of soil to enable the placing, grading and compaction of existing materials. In general, CKD is not placed with a spreading truck, or mixed with a rotary mixer. Special handling methods and precautions are required for placing and mixing CKD. The percentage of CKD added to the existing material is dependant upon the amount of moisture reduction needed to obtain workable, compactable soil.

Cement Treated Base (CTB)
CTB (soil cement) is a highly compacted mixture of soil, aggregate, cement and water. It is used as a low cost pavement base for roads, residential streets, highways, parking areas, airports, shoulders and material-handling storage areas. CTB provides great strength and durability, and lessens the need for thicken pavement. CTB must be mixed with a rotary machine, and then applied using a spreading machine. It is then compacted to a specific density.

Soil Stabilization (CKD)
Soil stabilization is a procedure using Partially Calcined Material (Cement Kiln Dust) as a drying agent to achieve specific compactions when properly mixed with high moisture content soils. This approach, when used in conjunction with the correct product handling BMPs (Best Management Practices), provides a viable and effective earthwork solution for construction projects with wet soils.

Laboratory testing has confirmed that the use of CKD additives will reduce the moisture content of a wet, incompatible soil and improve its consistency to the point where it can be compacted to produce a stable, bearing subgrade.

This is achieved through two separate processes:

1. Chemically. The overall moisture content is reduced by the oxides in CKD reacting with water in the soil, in a process known as hydration, to bind the water molecules into a solid form. The "drying" of the soil does not occur simply because you are adding another dry material (diluting the moisture content and displacing it over a larger amount of material). Instead, an actual chemical reaction occurs which binds the water into a solid matrix. This reaction also decreases the level of permeability of additional moisture, which is extremely important when dealing with seasonal climate conditions or high groundwater levels.

2. Physically. Due to the extremely fine nature of CKD, (similar to Portland cement), the optimum moisture content of a soil will increase once treated with CKD. Take for example a glacial till soil with a moisture content that is 4% over optimum. By treating with CKD you may decrease the overall moisture content by 3% and increase the optimum moisture content by 1%, effectively handling the 4% over-optimum moisture. This is an important point, because CKD's ability to change optimum moisture content is often overlooked when compared to other soil stabilizers such as Quicklime.

Strength - Cement Kiln Dust has the ability to increase compressive strength in treated soil, this of course is greatly dependant upon on the amount of CKD added to the soil. While CKD will not add strength to the same degree as Portland cement, in many applications, depending on the strength requirement, CKD is still an effective alternative due to the cost savings over cement. In many cases, a blend of Cement Kiln Dust and Portland Cement are an effective alternative for strength and drying ability - with the advantage of cost savings over straight Portland Cement.

Economic benefits

The economic advantages and benefits of using Cement Kiln Dust or blends of CKD & PC are significant. The ability to utilize on-site material over importing/exporting impacts cost, resources, and time. The cost of the soil treatment can add considerable dollars to the project but will greatly reduce the traffic for importing and exporting through sensitive areas. Sometimes the additional strength added to a subgrade will result in cost savings, as it can reduce the amount of final surfacing needed later.

Delivery and application
Partially calcined materials are delivered to the job-site in pneumatic highway trailers. CKD is pumped into mixing pits or spreader trucks. Mixing pits are to contain, and mix the soil, and greatly reduce the amount of dust generated from the mixing process. Mixing can be achieved with excavators, rotary mixers, and dozers. The excavators are the method of choice for cut and fill applications, whereas, spreader trucks and rotary mixers are commonly used for final lifts, roads and pavement bases. After mixing, the treated material from the pit, can be moved into place, spread, and compacted immediately.

Blend ratios - Typical blend ratios for mixing CKD into wet soils is approximately 1% of additive for every .4% -.6% of moisture over optimum. The amounts to be used are determined by a soils engineer.

Soil types - CKD will work with almost all soil types including glacial tills and similar granular; and silty, sandy soils. CKD will work in clay soils also, but its higher plasticity index requires a higher blend ratio.

Environmental Impacts and Best Management Practices (BMPS)

Dust - Due to the fine, powdery, properties of CKD and Portland Cement, extra precautions are necessary to prevent any amount of airborne release. While unloading from pneumatic tankers, tarps and plastic is placed over the entire pit, as well as the discharge hose. The truck driver or operator must understand the proper operation of the tanker truck to maintain a clean unloading. Persons involved in tanker operation will follow specific instructions. In high wind conditions the operation is restricted to prevent the release of emissions from the unloading. Caution is also used when moving the dust to another mixing site.